The Original part VDO 360 081 029 030 is now obsolete and neither GW or QRG have any old stock left. I got in touch with Demon Tweeks (Nigels Recomendation) and they were able to supply an alternative part to the same spec, details are:

VDO 360-081-029-020C presure sender 10 bar (earth return) 1/4 - 18 NPTF M4 knurled nut.

Contact at Demon Tweeks is Mr Dave Featherstone on 01928 663042 (direct dial) or email davef@demon-tweeks.co.uk

Many thanks to Mark (MB owner for the tip above)